How can you tell when a self-locking nut must be discarded?

• A. When it is corroded
• B. When it requires a wrench to begin threading
• C. When it is loose
• D. When you can screw the nut onto a bolt without having to use a wrench

Answer: (D)

Self-locking nuts rely on a locking element, such as a nylon insert, to create friction that resists loosening under vibration. When that locking action wears out, the nut can be threaded onto the bolt with little or no resistance, meaning the friction that keeps it from turning has been lost. That loss of locking capability is the signal to discard the nut, not merely its ability to thread on. Corrosion or looseness can be concerns, but the definitive sign of needing replacement is being able to screw the nut onto the bolt without a wrench or without encountering the locking friction.